The first season is kind of variable - it ranges from good to atrocious.
The second through sixth are generally very good, although they do pitch a couple duds.
The seventh must be avoided at all costs.
Some guidelines if catching an individual episode on TV…
If Charlie is in it, it may be good, unless he’s the central character, in which case, skip it.
If Richie is in it, and not an Immortal, yet, watch it, but be prepared to roll your eyes.
If it deals with the Watchers, and anyone other than Joe Dawson or Adam Pierson is central, skip it.
If it’s in Paris, watch it. If it’s in Seacouver, then you’ll need to watch for other telltales.
If Methos, Fitz, or Amanda are in it, watch it. If two are more are in it, watch it, and have some extra pants on hand. >_> There is one exception to this - there’s a ‘It’s a Wonderful Life’ type episode in the 7th season which features all 3 (plus Richie). It’s the only watchable episode of the season, but it’s not really great.
If anything mystical other than the Immortals themselves comes up, skip it.
If it doesn’t have a recurring mortal character in it, watch it.
If Duncan’s girlfriend is Tessa, you can give it a miss, but probably don’t need to.
If it’s Anne, watch it.
If it’s Amanda…well, it means she’s appearing, doesn’t it?
If the name Ahriman comes up, turn it off, and burn your TV.